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I am having my two A/C units replaced with two 2-ton Infinity systems. In order to get more air into a couple of rooms that were always hotter with the old system, my HVAC tech is replacing the current ductwork going into those rooms with larger ducts, running them directly from the air handler to the registers. The concern I have is that he is not replacing the registers, and is running the larger 7-inch ducts into the existing 5-inches registers, joining 7" into the 5" near the registers. I know that this is not the right way of doing it, but should I be concerned with this set-up?

That's not necessarily wrong. The larger duct will carry a larger volume of air to the end of the run. Where it reduces, that volume of air will increase in velocity causing greater throw into the room. This is based on a relatively balanced system that is working at the correct static pressure. Basicly a properly sized duct system. If the larger duct doesn't have enough air feeding it, it will still have poor airflow.

Whoosh

Wouldn't reducing from 7" to 5" right at the register boot create a bit of noise? Or at the least, restrict the air flow substantially? My ductilator seems to think so. berad's point about the size of the trunkline is very valid also.
Run a load on the room, measure how much air you're getting and compare it to what the load says you should have. Don't let your guy guess!(It kinda sounds to me like he might be.)

7-5 ...

Increasing the 5" to 7" is going to increase the air flow reaching those registers. Nobody should be worrying about velocity and noise on a 7" duct run reduced to 5" at the boot, at best it will improve the throw of the register. Increasing the boot may actually dimish the throw the 7" provides to the 5" ....

I would not worry about noise of 160 cfm @ an estimated 600 FPM, the old 5" under perfect conditions provided 70 cfm @ 500 FPM not a real noticable difference.... An improvement but nothing to worry about...

the proper way to do it is run a trunk line with the correct cfms and then run short flex runs off it
flex just adds to the static pressure of the system
the reducers wont hurt any thing but be a little noiser
running flex from the pleumun i disagree with

If you trust the contractor, go with his recommendation. Its its screwed up and he is somewhat respectable, he will fix it. If there is any significant duct problems though, the infinity control will tell on them.